1. Field of the Invention
The present invention relates to polymeric molding compositions, especially to molding compositions comprising a pair of thermoplastic polyesters having different chemical structures. More particularly, this invention relates to polyester molding compositions in which the polyester constituents do not undergo a transesterification reaction to any significant degree when heated to the molding temperature.
2. Description of the Prior Art
To date, polyethylene terephthalate has been considered the most interesting of the thermoplastic polyesters from an industrial and commercial standpoint. However, polyethylene terephthalate, although readily available and having an advantageous price, has been accepted only to a limited extent as a molding resin for the manufacture or production of shaped articles. This restricted use may be principally due to the low degree of crystallization which is inherent in its structure, its tendency to undergo considerable shrinkage and thus resulting in distorted articles, and its tendency to form articles which are liable to change as a result of crystallization when reheated. The use of extended residence times in the mold to effect the required crystallization is of no industrial interest.
Certain polyalkylene terephthalates other than polyethylene terephthalate, such as poly-1,4-butylene terephthalate, have been used successfully as molding resins as a result of their specific ability to crystallize more rapidly than polyethylene terephthalate, which makes molding easier; demolding is greatly facilitated and cycle times are short.
However, poly-1,4-butylene terephthalate tends to be more costly than polyethylene terephthalate, which has in some measure restricted its development. Moreover, the resultant molded articles exhibit poor dimensional stability when hot as a result of the low glass transition temperature of this polyester (in the region of 40.degree. C.). As a result, the remaining mechanical properties, in particular flexural modulus and strength, also leave something to be desired at elevated temperatures. In contrast, polyethylene terephthalate molding materials produce articles which have a satisfactory combination of mechanical properties because their Tg is in the region of 75.degree. C.
Other attractive thermoplastic materials have appeared on the market; these are wholly aromatic polyesters resulting from the polycondensation of diphenols, such as, for example, bisphenol A, with aromatic dicarboxylic acids, such as, for example, terephthalic acid, isophthalic acid, or mixtures thereof. The change to a wholly aromatic structure results in an increase in the Tg in the region of 170.degree. C.), which in turn results in better retention of mechanical properties at elevated temperatures by articles based on said wholly aromatic polyesters. The development of these molding materials is considerably retarded, however, by the fact that the wholly aromatic polyesters are difficult to mold, in comparison with polyethylene terephthalate and poly-1,4-butylene terephthalate. Thus, for example, in injection molding, a wholly aromatic polyester requires a high molding temperature of approximately 300.degree. to 350.degree. C., a high injection pressure of approximately 100 to 150 Mpa and a high mold temperature of approximately 120.degree. to 150.degree. C.; the molded articles then have faults such as shrink marks, flow marks and high internal stresses.
It too is known to this art that the various faults referred to above, which are inherent in the structures of polyethylene terephthalate, poly-1,4-butylene terephthalate or of a wholly aromatic polyester, can be completely or partially corrected by utilizing mixtures of these polymers. For example, the addition of poly-1,4-butylene terephthalate to polyethylene terephthalate (which is present in major amounts in the mixture) will provide an improved polyester composition capable of being molded using residence times in the mold which are practicable industrially (cf. particularly French patent application No. 74/31,228, published under No. 2,243,978). As another example, the addition of polyethylene terephthalate to poly-1,4-butylene terephthalate (which is present in major amounts in the mixture) will provide an improved polyester composition which, when molded, will provide articles having satisfactory thermal behavior (cf. again the aforesaid published French patent application). As still another example, the addition of polyethylene terephthalate to a wholly aromatic polyester (which is present in major amounts in the mixture) will provide a composition having very clearly improved moldability (cf. particularly French patent application No. 73/23,798, published under No. 2,190,873).
It is also known that, when two polyesters having different chemical structures are combined in the melt to produce a mixture, it is to be expected that the polyesters undergo a transesterification reaction on being heated, which can be discerned by the appearance of a copolymer, by a marked lowering in the melting point of at least one of the polyester components, and by a considerable reduction in the rate/degree of crystallization thereof. On being molded, the polyesters which are partially transesterified in this manner produce articles having less attractive mechanical properties than those produced from non-transesterified polyesters. Consequently, the use of mixtures of polyesters in which transesterification occurs is not particularly recommended, especially when a long residence time in the mold is considered to be important and when the production of good mechanical properties and their retention at high temperatures are deemed to be important. Cf. U.S. Pat. No. 4,123,420, published French patent applications Nos. 2,223,424 and 2,243,978, and German Offenlegungschrift No. 27 49 261.